Taxes and the related administrative structure are among important issues in the study of economic history of medieval Iran. Fiscal and tax events were recorded in special documents; therefore, studying and classifying them is the first step in clarifying the administrative processes of taxation and its accountings. Taxation expressions and documents are described in a number of texts before 10th century A. H (15th century A. D). Even though there are a few studies on taxes, there is no independent study on tax documents. This paper, with the use of explanatory method in historical research, has tried to answer this question: What was the structure of tax documents and books in administration system of medieval Iran? Findings show that documents, based on priori and posteriori relation with taxation events, can be divided into two main groups of reference documents and current documents. Reference documents, containing budget documents and statistical and cadastral books, were used as a source for taxation activities. Current documents were various books and reports assigned to recording, classifying, and summarizing fiscal events and their results.